he objective of this project is to identify immunoaccessible urface antigens of Chlamydia trachomatis organisms that are otential candidate antigens for the development of a hlamydial vaccine. The study has focused on two acromolecules, the major outer membrane protein (MOMP) and ipopolysaccharide (LPS). Monoclonal antibodies have been aised against each of these macromolecules. Antibodies (IgG3) hat recognized type-, subspecies-, and species-specific pitopes located on MOMP and a genus-specific epitope located n LPS have been identified by immunoblotting analyses and ndirect fluorescent antibody. The immunoaccessibility of hese antigenic determinants on viable chlamydiae and their iological significance have been studied by the following ethods: (i) binding of radioiodinated antibodies by viable rganisms, (ii) immunoelectron cytochemistry with protein A ollodial gold as a probe, and (iii) neutralization of in vitro nfectivity. The findings show that only the type-specific ntigenic determinant is immunoaccessible on viable hlamydiae. Epidemiological data indicate that either mmunization or natural infection confers partial immunity, and his immunity is restricted to the infecting C. trachomatis erotype. These observations suggest that the peptide fragment f MOMP which contains the type-specific determinant would be a easonable candidate for a subunit peptide vaccine for C. rachomatis infections. A 15Kd peptide with these antigenic roperties has been identified from CNBr digests of MOMP Project Z01 AI 00233-03 LMSF) and the nucleotide sequence of he MOMP gene that codes for this determinant has been cloned n Escherichia coli. The function of MOMP and LPS in hlamydial attachment, penetration, and inhibition of hagolysosome fusion is also being studied.